The present invention relates generally to the field of switch mode power supply (SMPS). More particularly, embodiments of the present invention relate to circuits and method for suppressing audible noise in switch mode power converters.
Electronic and magnetic devices, such as a switch mode power supply (SMPS), can generate audible noises when the switching frequency is in the audible range, e.g., below 20 kHz. Under many conditions, audible noise is often not acceptable. In pulse frequency modulation (PFM) converters, audible noise can be generated under low load conditions. In certain applications, audible noise can be reduced using optimization at system level, such as by lowering the maximum flux density of the transformer. Alternatively, audible noise can also be reduced using techniques at chip level.
Conventional chip level solutions are often not satisfactory. For example, U.S. Pat. No. 6,011,361 describes a buck converter for igniting and operating a high-pressure discharge lamp, in which the maximum off time of the transistor switching the converter can be set preventing operations below 20 kHz. These limitations are set all the time irrespective of if the switching frequency is high or low. Specifically, the off time has an upper limit of 36 μs and a lower limit of 5 μs. With regard to audible noise, the switch is turned on if a time limit for the off time is reached.
In another example, U.S. Pat. No. 7,202,609 describes a fly-back converter, in which the frequency of the transistor controlling the converter is monitored by using a timer to measure the period of the switch transistor, and the switching frequency is prevented from falling below 20 kHz. When the frequency reaches an audible level, the frequency is increased by turning off the transistor sooner, i.e., at a lower current level, by adding an extra voltage to the voltage across the current sense resistor. The extra voltage is provided by an internal current source.
These conventional techniques tend to be inflexible, unable to handle different load conditions, and often increase the cost of the system. Therefore, improved techniques for suppressing audible noise in switch mode power supplies are desirable.